The present invention relates broadly to the protection of electronic and electric components by encapsulation in plastic.
Electronic devices encapsulated in plastic are produced and used in the electronics industry in vast quantities. Such devices include, for instance, integrated circuits, transistors, resistors and diodes. They are produced in complex molding presses having precision molds and auxiliary loading equipment. The molds particularly are extremely complex. They are of metal and, to produce, require an extreme amount of labor by highly skilled workers. The molds have cavities therein for producing a multiplicity of encapsulated devices per press operation. To increase the number of devices produced, the molds must be made bigger, requiring bigger presses. The end result is that the production of encapsulated devices has become most costly because of the encapsulating step.
In the prior art, encapsulated electronic components, known as integrated circuit packages, or "IC's", are produced in great quantities. These can be for instance resistors, capacitors, or any other of many electronic devices. The encapsulating material is generally a thermo-setting material, such as epoxy or silicone.
The molds which are used to encapsulate these devices have been of metal. In order to withstand the high temperatures and molding pressure, these metal molds have been refined and highly developed to the present state of the art. Such molds are highly accurate, extremely strong, and extremely expensive. They do the job well but nevertheless have certain shortcomings.
A first shortcoming is that these metal molds are extremely expensive to produce, requiring many man-hours of skilled labor.
Another shortcoming is resin bleed, either caused by inaccuracies in the mold or from the product lead frame. In other words, the metal mold being extremely rigid will not compensate for discrepancies in the lead frame thickness and variations. Additionally, unless the metal mold is extremely precise in makeup, imperfect mating surfaces permit resin bleed. Thus, the finished encapsulated product has undesired plastic flashing or protuberances.
Another shortcoming in the metal molds is that to eliminate flash in certain areas, prohibitively expensive or impossible machining is necessary. Hence, the finished encapsulated product normally has flashing between the encapsulation and leads which must be subsequently removed.
All efforts to improve, and there have been many, encapsulation of IC's have been directed toward the improvement of metal molds.
Encapsulation systems usually consist of a molding press, a precision mold, and loading frames. The frames support the bare unencapsulated parts and carry such parts into the mold. Such machinery is extremely complex.
The components are encapsulated in a plastic, such as epoxy, silicone, polyester, alkyd or other materials. Such encapsulation involves a long-existing production problem. Evolution of concepts and equipment for protecting components has been a steady development, and particularly as regards delicate and fragile components such as integrated circuits, hereinafter referred to as "IC's".
As techniques and improvements have evolved over the years, materials of recent use in processes for encapsulating electronic components and the like, by the transfer molding process, have included epoxy molding compounds and silicone molding compounds.
Some of the characteristics of the molding materials used to protect delicate IC's, as presently envisaged, permit higher temperature operation and they are self-extinguishing or non-burning. The materials must be dimensionally accurate, chemically clean and inert. Care must also be taken to ensure that the molding compounds contain very few chemicals which can attack and corrode the subassembly of a semiconductor.
Materials used for encapsulation include epoxy and silicone molding compounds. Prior art methods of molding include compression molding, transfer molding and injection molding. Transfer molding, however, is the predominant process, and epoxies and silicones are the main molding compounds used. Products molded include, for example, coils and relays; capacitors and resistors with round wires or lead frames; and devices on metal stamped lead frames. Of increasing use are devices produced on metal lead frames. Basically, the process for an IC is to bond the IC "chip" onto a platform by gluing with a conductive material or by forming a eutectic bond of gold/silicon. The pads on the chip are attached to leads on fingers by very fine wires, for example gold or aluminum wires, as small as 0.001" (1/3 of a human hair in diameter), or smaller. The device must now be protected and provided with an appropriate shape, strength and identity.
Typical electronic component subassemblies consist of multiple connections of fine, delicate wires. The encapsulation techniques must allow a molding compound to enter a molding cavity, within which the electronic component is placed, from a runner, in a liquid form with close control of speed, pressure and viscosity, so as to eliminate damage to these delicate wires. To do this, the molding compound must, among other things, move easily from a solid to a thin liquid. The process must be controlled to create correct pressures, velocity and viscosities. The end result must be an encapsulation of the device in a solid, void-free package, without breaking or damaging the wires or the device itself.
In transfer molding, the molding compound, in powder, preform or preheated preform, is placed in an area of a closed mold called a "transfer pot." An auxiliary ram, or plunger, applies pressure to the surface of the molding compound, causing it to compact, soften and liquify. The heat and pressure cause this liquification. Continued pressure causes the liquid plastic to flow from the pot down a series of runners and into a cavity through a restriction known in the art as a gate. When the molding material fills the cavity, having surrounded the part to be encapsulated, the molding compound "cures" or hardens. Normally, such a process requires 1 to 3 minutes. In semiautomatic processes, as currently in use, the molding press opens, the parts, runners and any material remaining in the pot area are ejected by built-in ejector pins. The "shot" is removed, cull and runner separated, and discarded. The existing technique most widely used for electronic component encapsulation as mentioned above, is transfer molding.
In transfer molding technique, a molding press is used to contain the top and bottom parts of a mold which, typically, has numerous component cavities, and is used to transfer the molding material into the mold cavities. The press applies basically two forces. One force is used to clamp or keep the two halves of the mold closed. Presses can range in tonnage from 5 to 300 tons or more, with normal encapsulation range being approximately 10 to 200 tons, and second enormous forces applied to an auxiliary ram, or transfer ram to transfer the molding material into the mold cavities. An unprotected electronic component, which is positioned over a cavity in the bottom half of an open mold, can be readily damaged in the absence of proper apparatus. Subsequent to this placement, the press and mold halves are closed and the mold is heated within a range of approximately 300.degree.-400.degree. F. The molding compound is introduced, either as a powder or in a compacted shape called a preform which is a pre-measured amount. The preform is often preheated by means of an external, high frequency induction preheater. The compound, or preform, is positioned in a receptacle called a "transfer pot." The pot bottom, in operation, opens to a series of streets, channels or runners that lead to gates or openings in the cavity. The cavity defined by the top and bottom mold halves determines the final molded shape of the finished product. Subsequent to introduction in the transfer pot, the compound is compressed by the transfer ram. The extreme high pressure and heat of the mold causes the mold material to soften and melt, so that the liquid flows down the runner into the cavity, surrounds the component to be encapsulated and, because of the chemical makeup of the compound, it being a thermo set, undergoes an irreversible chemical process. The compound cures or polymerizes. The cycle time normally takes 1 to 3 minutes with currently available materials. Upon completion of the cure, the transfer ram retracts from the mold, the press and mold halves open, and the molded product, complete with runner and sprue, are removed, frequently with the assist of internal ejector pins or mechanisms.
Normally, the underside of the mold top and the topside of the mold bottom are then brushed and cleaned with an air blast to remove residual plastic or flash before placing new devices in the mold for encapsulation, following which the cycle is repeated.
As will be apparent, the molds used are extremely complex, very precise, expensive and difficult of construction, with formulation and delivery involving a substantial length of time.
The technology of encapsulation by transfer molding is ever expanding and developing. At the same time, the encapsulation molds are developing. Molds are known today which will encapsulate, for example, 120 14-lead IC's. Such molds must incorporate extreme accuracy to ensure accuracy in the final molded package, and additionally the mold must seal off the lead frame in a manner to prevent damage to the frame and leads while preventing undesired molding compound seepage. The molds must be built to extremely close tolerances. The metal of the molds must additionally be of very high hardness to prevent the compressive force of the press from damaging the mold's surface.
The current molds in use, such as a 120 cavity mold for a typical 14-lead IC, can require in excess of 1000 man hours to build by highly skilled tool and die makers. A typical 120 cavity IC mold has an extremely high selling price in view of the complexity and time involved in creation of the molds. There is a continued trend to use molds having greater capacities and, accordingly, a greater number of cavities. Under the present existing cost factors involved, a mold of 400 cavity capacity could cost in excess of $100,000.
In light of the foregoing background and history regarding development of micro-electronics, and the encapsulation of electric and electronic components within plastic, the ever-increasing mold costs and complexity are of extremely great significance. More and more types and numbers of parts and units are in demand.
The present invention is directed to a very substantial contribution to overcome these ever-increasing problems and, to this end, a new system, process and apparatus are taught. The invention results in a very substantial reduction in mold and molding costs, utilizes less molding compounds, reduces mold complexity, reduces delivery times for the equipment and increases the productivity of the transfer molding process in general.